CNC Milling equations... and how I can improve Gantry rigidity

Mostly Printed CNC - MPCNC Troubleshooting
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I’ve been having untold amount of trouble with my MPCNC recently. 8 endmills broken now. My butt is being kicked by this machine, bigtime.

Questions:

  1. Can someone share with me ALL of the key equations I need to calculate Feeds/RPM/DoC in advance?
  1. What conservative chipload value should I aim for each ‘material - endmill combination’?
    e.g. Brass - 1/8" single flute endmill - chipload ___
    1/16" single flute endmill - chipload ___
  1. Advice on further improving rigidity of MPCNC gantry would be ideal coz no matter how much I tighten all the bolts, I can still find a small amount of rocking motion in the gantry when I manipulate it by hand. I’m out of ideas. Rigidity improvement suggestions anywhere and everywhere on the mpcnc would be great.
  1. Is airblast really THAAAT helpful when you’re only cutting 3mm metal stock?

My MPCNC:

Working Area Dimensions - 700mm (L) x 350mm (W) x 50mm (H, reduced to 25mm now by raising the spoilboard).
MPCNC Part types - ‘F’ variety.
Print infill - 70%, Adaptive Cubic
Conduit Diameter - 25mm, Hollow.
Router - Makita700.
Endmills - All V1 variety (mainly been using 1/8" single-flute endmill).
Airblast - None.
CAM Software - Carbide Create (Doesn’t give ‘trochoidal/Adaptive milling’ option fyi).
Stock thickness - 3mm (not used any other thickness so far).

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Just a quick response from my side: sounds like 700mm is a little bit on the longer side, especially for milling metals… ?

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You could try something like this: Feed Rate Calculator, but most of the time they are for commercial CNCs. I bought endmills that had came with a booklet, the speeds in there are not possible with the MPCNC. It’s more about trial and error. Start low (or copy the speeds of a more or less known youtuber) and work your way up. None of us will be able to give you the equations I guess, because all of our machines are different. :frowning:

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Airblast or vacuum will help cool the endmill, and keep from recutting chips. You can use a feedrate calculator for a Shapoko. That’ll get you close enough to adjust from test cuts. https://docs.carbide3d.com/support/supportfiles/S3_feeds_250.pdf

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I think the only things i didn’t mention in the other thread (about formulas and cut recipes being problematic on hobby machines) is that these machines will be a lot more rigid in a corner, and that you can mount your stock on some boards to raise it up closer to the gantry. As long as the z tubes engage the lower rollers, you’re good to go.

Something else to consider is grabbing the endmill (machine OFF, of course) with gloves if necessary, and giving it a wiggle. See if there is any appreciable flex, and try to track it down.

All of my new machines loosened up after a week or so. I’m not sure if it’s the plastic, the heat out here, or just the way I work. A friend of mine just built a primo, and I trust his skills a lot more. His also loosened up just sitting in the work shop.

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Another thing about feed and speed numbers found online, besides missing variables like machine flex, they also leave out some more general yet important variables. For example cutting depth and step over are often not listed, nor is mill length and flexibility. The latter flex alone can be hugely different between a common 1” long 1/4” upcut, vs a 3/4” long 1mm upcut. Even in a Bridgeport mill, the numbers for the 1mm upcut will have to be a lot more conservative due to bit flex.

Also for metals, unless you have a very slow turning router, any help you can get to push feed rates down is welcome to prevent breaking bits. Less feed means more heating. So mist is pretty common for routers cutting metal.

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FYI Everybody - the problem turned out to be lack of rigidity in the stock.

The double-sided sticky tape was trash on the MDF spoilboard. Carpet tape solved the problem and my results have been absolute perfection for the past 5 test cuts I did, even at the most demanding settings I’ve done so far. No Chatter, absolutely crisp edges and valleys of cuts. Just perfection.

Now I know what the problem is, I’m devising a removable spoil board permanently for machining metals where I:

  1. stick a plate of aluminium to the removable spoilboard using the carpet tape, and then
  2. using the double-sided sticky tape, stick my metal stock to the aluminium ‘spoil board’ surface (coz the double sided tape sticks great to polished aluminium stock, but will be easier to separate than the carpet tape after finish cutting).

Wish me luck, coz now my MPCNC has decided it doesn’t want to turn on anymore… lmfao.

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